1. Field of the Invention
The invention relates to television receivers.
Currently the majority of television programs transmitted have a nominal picture aspect ratio of 4:3, that is, the picture is four units wide and three units high. As a result, most television receivers are provided with a display screen having the same aspect ratio. However, many programs are made with a different aspect ratio, particularly feature films which were originally made for display in Cinemas. These programs are frequently broadcast in a form which has become known as "letterbox format". This results in black horizontal bars being displayed at the top and bottom of a normal 4:3 format display screen.
2. Description of the Related Art
An increasing number of television receivers are now being produced with a display screen having an aspect ratio of 16:9. This more closely matches the normal film format and reduces or, in some cases, eliminates the black bars at the top and bottom of the screen enabling the picture to occupy the full screen area. However, broadcasters often use the lower black bar to insert subtitles. If the transmitted picture is then expanded to fill a 16:9 aspect ratio display, the subtitles are lost. One solution to this problem is to raise the picture on the display to bring the subtitles back into view, but this causes loss of the top of the picture.
The FIGS. 1a-1f illustrate possible display formats on a 16:9 aspect ratio display. FIG. 1a shows the traditional 4:3 aspect ratio picture with black horizontal bars at the top and bottom of the picture just as it would appear on a normal 4:3 aspect ratio display and also blank vertical bars down each side of the picture which fill the unoccupied left and right hand areas of the 16:9 display. In this case the subtitle is shown in the lower horizontal bar. If the picture is then expanded to fill the display screen as shown in FIG. 1b then since the horizontal black bars are lost as well as the vertical bars at each side of the display, the subtitles are also lost. In order to recover the subtitles, it is possible to raise the picture, as shown in FIG. 1c, to restore the subtitle at the bottom. This will, of course, result in the top lines of the picture being lost. This option has been implemented in currently available 16:9 display format television receivers. The problem of the loss of picture content is exacerbated in the letterbox format shown in FIGS. 1d-1f where, in order to more easily accommodate more than one subtitle line, the letterbox format picture is already raised to the top of the 4:3 format display screen. That is, the broadcaster, knowing that the program will in most cases be viewed on a 4:3 aspect ratio display, transmits picture information from the first active line of the display thereby leaving a single, double height, black bar at the bottom of the display thereby allowing multi-line subtitles to be accommodated wholly within the single black bar. Under these circumstances, as can be seen in FIG. 1f, the degree of loss of picture content becomes significant, and in some instances unacceptable, particularly if the shifted display includes the whole of the black bar which potentially contains subtitles.
It is also known to take subtitles from the black bar at the bottom of the screen and to raise them to superimpose them on the picture area. Thus instead of raising the picture as shown in FIG. 1c and FIG. 1f, it is retained in the position shown in FIG. 1b or FIG. 1e but the subtitles are superimposed on the displayed picture. Television receivers having such a feature have been sold in Japan but Applicant is presently unaware as to how this has been achieved. One possible way to achieve this function is merely to add the luminance values of the lower black bar and the subtitles it contains to that of a lower horizontal section of the picture. This will make the black bar effectively transparent and give high luminance subtitle characters.
With television receivers having a 16:9 aspect ratio, a significant proportion of the program they presently receive are transmitted in a 4:3 aspect ratio form. These can be displayed with vertical black bars down each side of the display area but viewers may wish to expand the picture to fill the display screen. This inevitably means that some of the picture is lost unless the expanded picture is distorted by expanding only in the horizontal direction. Such a distortion is not believed to be acceptable and consequently, in presently available television receivers, the expansion is carried out equally in both directions and the loss of part of the picture is accepted. Normally this will be a narrow band at the top and bottom of the picture and frequently this will result in little significant loss of picture information and any such loss may be acceptable to the viewer, particularly where the viewer is given the choice as to which format to use on any particular occasion and the freedom to switch from one to the other instantaneously. Where subtitles are transmitted, however, they are likely to lie in the part of the picture which is lost and consequently will not be available to the viewer. A solution to this problem, which has been adopted in practice, is to provide a control which allows the 16:9 window to be placed anywhere on the 4:3 picture. Thus, to ensure display of the subtitles, this window can be placed at the bottom of the picture. As a result, however, the loss of information at the top of the picture becomes more noticeable, particularly in scenes with significant information content at the top of the picture.